Cutting Through the Hype About 56K Modems

This article was first published in September 1997 when two different protocols for 56k throughput, X2 and K56flex, were competing. Starting in March 1998, v.90 was developed to replace these competing protocols and provide a single standard for 56k modems. v.90 was finalized in February 1999. This article was last updated at about that time, and most links are now to pages in the Internet Archive.

Since then, the v.92 standard was developed to allow uploads at speeds up to 48 kbps – but at the expense of download rate. We discuss the trade-off between upload and download speeds in this article, which has not been updated to reflect the newer standard.

  1. Not all phone lines will support 56k modems.
  2. FCC restrictions may limit you to 53kbps.
  3. Real world connection speeds are under 56k (but often better than 33.6).
  4. 56k speed is for downloads only; uploads limited to 33.6kbps.
  5. Upload speed is inversely proportional to download speed.
  6. 56k modems don’t talk to each other at 56k.
  7. 56k may be impractical for remote access.
  8. Latency – why some aspects of 56k are no faster than with your current modem.
  9. Data buffering and compression – two more factors to slow things down.
  10. Serial port speed makes a difference.
  11. What about faster Internet connections?
  12. Good things about 56k modems.
  13. Conclusions.

1. 56k modems will not work at high speed everywhere. US Robotics has set up a BBS where you can test your line for X2 support by dialing 1-888-877-9248 from a terminal program and logging in as LINE TEST. Be sure to visit their web page for full details.

If your phone line fails the test, you cannot use a 56k modem beyond 33.6kbps.

2. In the US, the FCC places a power ceiling on phone lines of -12dbm average per 3 second interval. X2 modems work within this by restricting throughput to 53kbps in the US. X2 modems can theoretically work at 56k, although they are constrained to operate 5% slower than this in the US. (Some users have reported occasional connections past 53kbps.)

Rockell reports that K56flex can achieve 56kbps performance within the FCC power restrictions. Regardless of protocol, you need exceptionally clean phone lines to reach top speed.

3. While v.90 modems may offer a download improvement compared with 28.8 and 33.6 modems, they are still a bit more expensive. Reviewers report typical download speeds in the 34-48kbps range. At the low end, it’s no better than 33.6; at the top end, it’s very fast – but your upload speed may suffer grievously (see point 5). Throughput is very much dependent on line conditions.

US Robotics’ Mindspring Test shows 4.3% of connections at 49.2kbps or faster, 61.4% at 44-48kbps, the remaining 34.3% connecting between 29.3kbps and 42.6kbps. (Most 28.8k and 33.6kbps modems rarely, if ever, connect at their rated speed.)

4. These modems don’t offer similar performance in both directions. They can only achieve 56k throughput when you are downloading files. Since this is what most of us do most of the time, that may not be a huge negative factor. 56k modems upload at a maximum speed of 28.8 or 33.6kbps (see point 5 for more details on upload speed).

5. The maximum total throughput (upload plus download) is limited to roughly 64kbps, just as it is on 33.6 modems. All three 56k standards (v.90, X2, K56flex) are asymmetric protocols. This means that a 48kbps download connection leaves only 16kbps of bandwidth for uploads. The faster your download, the slower your upload! (However, 56k modems also continuously monitor and adjust their speeds, so they can improve upload performance by temporarily reducing download speed.)

See benchmarks from Windows Source for more details. Add the best upload and download speeds for the barely compressible self-extracting archive in each test (33.3k, 37.3k, 48kbps, and 49.3k connections): They never add to more than 63.2kbps. These benchmarks show that the X2 modem doesn’t achieve upload speeds equal to a 28.8 modem at any of the tested download speeds. K56flex modems appear to work the same way.

If you are hosting a website, this can be a strong argument against adding a 56k modem. Outside users would be limited to whatever bandwidth is left after you establish a connection with your ISP.

6. Not only are the X2 and K56flex standards incompatible with each other, they cannot talk to other modems with the same protocol at 56k either – nor can v.90 modems. All 56k protocols require a complementary modem on the receiving end, one that can receive data at 56k. This means you cannot connect to another computer at 56k unless that computer has a receiver modem. Even if you have a set of 56k modems, remember that you’ll be running up to 53kbps in one direction and 33.6kbps or less in the other. Finally, the receiver modem must have a digital connection to the phone company.

7. If you wanted to use a 56k modem for remote access, read point 6 again.

The 56k connection requires a local phone connection with no more than one digital converter. The required 56k receiver modems are not as available as the client modems. Although receiver modems are being made, manufacturers want to get these into the hands of ISPs first, so dealers (local or mail order) are unlikely to stock them. They are also more expensive (US Robotics’ sells for about $450).

But the whole issue of download speed reducing upload speed (point 5) can make remote access with a 56k modem less efficient than with a 33.6 modem even if it is a local call and you have a receiver modem.

8. Some aspects of 56k modems are no faster than on the 33.6, 14.4, or even 2400bps modem you may already be using. (For instance, it takes about the same time to dial and connect regardless of modem speed.)

Although your modem can move lots of data in a second, there is a delay between your computer starting to send data and your modem beginning to transmit that data over the phone line.

In his article, It’s the Latency, Stupid, Stuart Cheshire contends that what we call modem speed would better be called capacity. A 28.8 modem has the capacity to transmit 28,800 bits per second; this does not mean it will always do so. This is partially because of the overhead of handshaking, which is rooted in protocols going back to the earliest data modems. That delay is called latency – and there’s nothing you can do about it.

For more details of latency, click here.

9. Another bottleneck is the way modems usually process data: one packet at a time. The modem waits until it receives a specified number of bits and then sends the data packet. If you use compression, there is a slight delay while the modem compresses or decompresses the data. This reduces throughput, since this data is sent in batches rather than as a continuous stream of bits.

Here’s what happens. Let’s say you’re using an external v.90 modem on a 115kbps serial port and packet size is 1000 bits.* The modem must receive all this data before it will process them. This takes 9 ms. Now the modem transmits the data, which takes at least 19 ms (assuming 53kbps connection). Fortunately, most modems have large enough buffers to transmit one packet while receiving the next one. Still, to send 1000 bits of data via modem will take at least 138 ms (latency plus input plus transmission). Overall throughput is 7.2kbps.

This assumes no compression. If your modem compresses data at 2:1, it will take 17 ms to receive 2000 bits (enough data to compress into 1000 bits). Add 1 millisecond for compression. (Cheshire notes that the compression/decompression chips in modems are far less powerful than the CPUs in today’s computers. Thus, it would make more sense to compress the data before sending it to your modem and turn off the modem’s compression.) Add 19ms to send the data. You’ve sent 2000 bits of data in perhaps 147 ms. Throughput is 13.6kbps, not quite double what it was without compression.

Third scenario: Your computer compresses the data at 2:1. This should take far less than 1ms, since today’s CPUs are so powerful. Add 9 ms to send the compressed data to your modem, 19 ms to transmit, and 110ms for latency, and you’ve send 2000 bits of data in 139 ms. Final throughput is 14.4kbps. Now you’ve finally doubled throughput.

For more details on buffers, compression, and latency at different modem speeds, click here.

* It probably isn’t 1000, but 1000 is an easy number to work with.

10. Buffering has its good side and bad side. We’ve discussed the bad side above. The good side is that the buffer allows your modem to transfer data to or from you computer faster than it can send or receive it over phone lines.

Data compression means your modem is moving data that is compressed before it is sent. This allows your modem to have a data throughput higher than its connection speed. That’s why it’s important to have the fastest possible serial connection. For instance, v.42bis modems can compress data at up to 4:1. You ideally want a serial connection 4x faster than your modem tot ale advantage of that.

In the era of 1200 and 2400bps modems, 9600 and 19.2kbps serial ports were fine. By the time 9600 modems came around, 38.4kbps serial ports were pretty common. For best performance, a 14.4 modem should be used with a 57.6 kbps serial port, although the older 38.4 ports can keep them running at or near capacity.

Enter 28.8 and 33.3kbps modems. You really want a 115.2kbps serial port to handle the potential data flow, although 57.6 kbps will offer decent performance. (Fortunately, higher serial port speeds have become more common in recent years.)

The new crop of 56k modems brings it a step further. Four-to-one compression means you want a 230.4kbps serial port – a number that should be familiar to longtime Mac users, as it is the speed of the standard Mac serial port and AppleTalk networking. A v.90 modem will work with a 57.6 kbps or 115.2kbps serial port, but you won’t get all the throughput your modem is capable of.

Personal testing with a Supra 56e modem, FreePPP, and both a Centris 610 and Centris 660av shows best throughput occurs with the serial port set to 230.4kbps. Dropping the serial port to 115.2kbps results in a 5% reduction in throughput. Moving to 57.6 kbps cuts 35-45% compared with the 230.4kbps setting. Top total throughput in my testing was 88.5kbps on a 230.4kbps serial port, 83.2kbps at 115.2kbps, and 52.7kbps at 57.6 kbps. (Modem connection speeds ranged from 31kbps to 44kbps.) Your results may vary.

11. What about faster Internet connections like ISDN, ADSL, and cable modems? Will they quickly replace v.90 technology?

ISDN is currently [1997] available in most of the US and offers 64kbps or 128kbps service. Unlike v.90 modems, ISDN requires a dedicated phone line (or two) and is considered a premium (read: high cost) service. Not all areas wired for phone service are wired for ISDN. The hardware, installation fee, and monthly service charges make this a very expensive option compared with v.90 modems.

I’ve been examining ISDN for personal use. Although Ameritech offers $122 line installation and about $53 per month for the connection, most ISPs want $200 and up for their end of the deal. On top of that, you need a relatively expensive ISDN modem (app. $200) or routers ($800+).

Cable modems are being deployed slowly. Obstacles include the fact that many cable installations are not designed for and cannot yet handle communication in both directions, many cable companies do not yet support it, and high cost (although the monthly fee often includes renting the modem). So far, users are ecstatic about bandwidth and the full-time Internet connection. For the most part, cable installations use the same kind of wiring found in coaxial ethernet networks. Bandwidth can be 10-30Mbps for downloads, up to 10Mbps for uploads. (@Home promises 768kbps uploads.) If several neighbors have cable modems, you may see speed drops while other users are online. Still, speed should be better than alternative technologies. Another downside: Many areas, particularly rural ones, aren’t wired for cable.

ADSL looks promising, but broad deployment is a year or more away. ADSL promises most of the speed of cable modems over regular telephone lines. If you have a phone, odds are pretty good you’ll be able to use ADSL – someday. For best speed, you must be within 18,000 feet (3 miles) of a switching station (and the switching station must be equipped with an ADSL receiver modem); the closer, the better. Promised speeds vary, but 640kbps uplinks and 6Mbps downlinks are commonly claimed speeds. If this can be successfully deployed, it will probably become the leading replacement for analog modem technology, followed by cable modems and ADSL. (ADSL can work on the same line and at the same time as your regular phone, giving you the capability for a full time Internet connection.)

There isn’t a lot of benefit going faster than 56k today [remember, this was written in 1997]. According to, “It’s the right amount of bandwidth.”

In a PC Magazine editorial, John Dvorak writes, “Even so, users with access to a T1 phone connection will soon discover that the fastest provider can send data at only around 56kbps – slower than a single B-channel over ISDN. This isn’t likely to change as providers try to serve as many users as possible, rather than pump 128kbps or more to a few people. So the ideal connection for internet surfing is a single B-channel on an ISDN line.” 56k modems have about the same bandwidth as single channel (64k) ISDN.

Many links along the Internet still use 56k frame relay. Once you get beyond today’s 28.8k and 33.6k modems, those frame relay links create the bottlenecks. Early testers of cable modems have already run into those bottlenecks. Says Dwayne MacKenzie, a member of a cable modem test community in Phrata, Pennsylvania, “The Internet just isn’t set up to handle everyone with 500-kilobit modems.” (Reprinted by permission of

In addition to latency, bandwidth is becoming a problem. Internet IT Informer reports testing of various backbones and found the fastest, Compuserve’s newly completed one, offered about 260kbps throughput; the slowest, Bell Canada, was 15kbps. The fastest is about twice as fast as a dual-channel ISDN connection; the slowest about the speed of a 14.4 modem. The rest fall in between, with the second fastest, GridNet, providing just half the speed of Compuserve.

Neither ADSL nor cable modems can overcome the limited bandwidth of the current Internet. Keynote/Boardwatch reports the average performance of the internet’s backbone was 40kbps – about what many users of 56k modems report as common access speeds.

This argues against investing in faster technologies until the infrastructure is improved.

New technology announced: Internet connection via power line announced by Nortel and Norweb. Promising 1 MBps full time Internet access without tying up your phone line, this could be an inexpensive alternative to ADSL and cable modems. Promised speed is 10-30x faster than v.90 modems (depending on compression and line condition) and nearly the speed of 10Base-T ethernet. Read more in PC World Online.

12. Good things about 56k modems.

  1. They may offer better performance than your current modem over the same line – especially if you are running 14.4 or slower.
  2. They are capable of downloading faster than 33.6kbps.
  3. They are reasonably priced.
  4. This may be the fastest modem technology that won’t require a dedicated phone line (like ISDN), multiple phone lines (bonded modems), or a special receiver at the phone company’s switching station (ADSL).
  5. Most 56k modems now support the v.90 standard.
  6. Much of today’s Internet offers about 128kbps performance. 56k with compression is enough bandwidth for now. (This will change over time.)


  1. If you have a 14.4 or slower modem, you’re overdue to upgrade. Buying a 56k modem makes sense. Be sure you buy a v.90 modem. If your ISP doesn’t yet support v.90 modems, you can use it as a 28.8k or 33.6k modem until then.
  2. If you have a 28.8 or 33.6 modem, don’t buy a 56k modem unless you’re a heavy modem user and can connect at 40kbps or better.
  3. If you need to buy a new modem, be sure to buy one that supports v.90. has a list of companies offering free upgrades to the final standard.

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